Published on 17 Feb 2014

Why are rivers still rising after the rain has stopped falling?

The most recent storm to hit our shores on Friday night caused yet more damage and disruption, taking the lives of two people.

Severe gales battered southern parts of the country, bringing down trees, damaging buildings and throwing large waves at the south coast.

In addition, further heavy rain exacerbated the flooding situation, falling on saturated ground before running off into rivers that are already high.

flooded_road_g_wp

I wrote in my blog on Friday, that the weather isn’t going to be as stormy this week, but despite this, the flood risk is going to remain.

Rainfall lag time

The Environment Agency’s three-day flood risk forecast still has parts of the Midlands and much of southern England at a medium to high risk of flooding during the next few days.

You’re probably thinking, why is this the case when the worst of the rain stopped falling on Saturday morning?

Well, when rain falls, it takes time to feed into rivers and even more time to flow downstream.z_3day_floodrisk_wp

Environment Agency flood risk forecast – low (yellow), orange (medium), red (high)

In the case of the River Thames, it can take around two to three days for rain water in the upper part of the river to flow downstream to the lower part of the river.

So, in effect, the rising of the rivers during today and tomorrow will be as a result of the rain that fell a few days ago.

Some rain this week

Although the general trend this week will is not as stormy, there will still be some rain at times. However, it is generally not going to be as intense as last week.

There will also be a trend for low pressure systems taking a more northerly track than last week. This will mean that the distribution of the heaviest rain will tend to be for northern and western areas, with southern and eastern areas relatively drier.

Looking further ahead, the jet stream during the next week looks to be weaker than during the last month, as well as snaking around more – rather than getting stuck in the same place.

This means that even though the weather will stay unsettled, spells of rain will be interspersed with drier, brighter days.

Even though the prospect of not as much rain may sound like light at the end of the tunnel, given the saturated ground, it will only take a little rain to see flooding issues return.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I’ll also be posting updates on Twitter – @liamdutton

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